Automotive clearcoats must be many performance requirements. They must be smooth and glossy to provide the desired aesthetic appeal. They must also be durable, both to preserve the coating appearance and to protect the steel substrate, by resisting scratching and marring and also degradation from UV light in sunlight, environmental etching, and heat. But the clearcoat has at least one important safety performance requirement, also. Windshields and other stationary glass are adhered to the coated vehicle frame with a bonding adhesive. The clearcoat must form a strong bond with the materials on either side to pass the federal motor vehicle safety standards (MVSS 212) Crash Barrier test, which tests, among other things, the crash-worthiness of the windshield and other stationary glass installation. In crash and roll-over situations, the windshield, rear window, and other stationary glass panels may provide additional support of the vehicle roof and must remain bonded to the vehicle body. In the past, a tape masking process has been used or a special primer has been applied between the clearcoat and the windshield sealant to obtain the desired bond strength.
Saracsan et al., U.S. Pat. No. 4,643,794, describe using a primer to join a clearcoated metal surface of the vehicle and the glass surface. The primer comprises a dispersion of a linear polyester resin, a polyisocyanate crosslinking agent, carbon black, and a volatile organic solvent. A sealant is applied to the primed surface and the metal and glass surfaces are then joined.
Baghdachi et al, U.S. Pat. No. 5,147,927, describes a moisture-curable polyurethane sealant that can be applied to electrocoat primed metal substrates without an additional primer layer. While the Baghdachi process does not require adhesion between clearcoat and sealant, it remains the general desire of automotive manufacturers to attach the fixed glass windshield panels over the clearcoat layer, not the electrocoat primer layer. The Baghdachi sealant composition is formulated specifically for application over the electrocoat primer and does not provide direction for achieving adhesion between a clearcoat layer and a glass sealant.
Considerable research has gone into making clearcoat more resistant to environmental etch-type degradation. Hsieh, U.S. Pat. Nos. 5,238,993 and 5,115,086 describe primer compositions that have been specifically formulated to be applied between environmental etch-resistant clearcoats and the glass sealant for the windshield. The compositions includes a polyester, a particular aromatic polyisocyanate, and in the '993 patent, a silane-containing moiety. The Hsieh patents do not provide direction for eliminating the need to a primer layer.
Eliminating the primer layer would be beneficial for simplifying the manufacturing process, as the steps of application and curing of the windshield primer could be eliminated, along with the equipment necessary for these steps and the regulated emissions resulting from these steps.